Ok, for starters, I have no science to back this post up. (And I don’t care.)
What I do have is 13 years on my indoor trainers, 5 in trainer road, so some historic data of progression.
I started out doing Spinervals DVD’s on my Kurt Kinetic trainer with Coach Troy (You can do it!). This was my introduction to actually practicing skills like cadence and form drills. It was great. Kept me almost fit.
After the Wahoo Kickr came a long, I wanted one but couldn’t justify the cost, but I discovered I could train with Virtual Power in Trainer Road by adding a speed and cadence sensor to my bike. $120.00 cost plus TR. Worked (and still works) great. I got into structured training, and learned a lot. Over 3 years my FTP rose to about 280.
Then I bought a used Kickr (now upgraded) and WOW it was different in ERG mode. I expected it would be harder. This was based on my previous data that clearly showed me fading on certain intervals during my programs. This fading is just how it is when you get tired as a weekend warrior. Sometimes I’d fade for a few seconds, sometimes I’d bonk the interval and just finish it low, but mostly I could not keep my sprint intervals on power.
So, I read a bunch about cycling theory to understand what I needed to change. The short version is: build your heart to pump more fully, build out your capillaries to move O2 to your muscles, then build out muscle power and speed.
(Read all of these fact sheets: EFR ~ Factsheets)
For the sake of this discussion, we’re talking about if ERG is better for building muscle power and speed. (Power = force (how hard you push) x speed (how fast you spin))
When I changed over to a Wahoo Kickr, I assumed that ERG mode would make me work hard for every full interval. No slacking off when I get tired. And yes, that’s what or does for me. I only like to train Low Volume (I do lots of outside winter activity) and it keeps me on target all the time. I feel that it helps me maximize my Low Volume training potential. It also allows me to focus on one aspect of generating power individually, or both together, or to focus on form.
My goal for the last 2 training season (Oct - May) have been to extend my FTP and increase my cadence. So, FTP endurance if that makes sense. I’ve seen my FTP test out now at 315 watts, but I don’t care about increasing it. It takes more work than I’m willing to put in. As a mountain bike instructor, I’ve found it more useful to be able to output 303w for 3 hours than 340w for 20mins. I’ve increased my usual cadence to 85 minimum with peaks over 120rpm. This gives me a lot of flexibility on how I generate watts when riding.
One counter intuitive thing I do is set my FTP slightly low in Trainer Road. I set it between 290 and 300 (not my tested 315). This keeps my target wattages in intervals a little lower than what I could peak at and I find it helps me to maintain and build higher cadences. It also means that I rarely bonk an interval. When I do want to work on slow cadence for climbing or standing, the slightly lower FTP saves my bad knees and hip for excess strain. I’m using ERG mode and I feel that I get good training results because I stay on power with high cadence in the complete interval even though the targets are touch reduced.
In Tour de Zwift this season consistently finished top 25% (top 10% a few times) in my categories, with output power over 300w for much of the ‘race’ ( note that I’m 100kg so there is no expectation of finishing first, but if the course of flat or rolling…)
I don’t think ERG has any real impact on muscle fiber develoment, other than the impact of getting you to work hard on target. But if you’re not completing the intervals and program with regular bonks, I’d suggest lowering your FTP to help you complete more.